1. Field of the Invention
The present invention relates to a liquid display device in which light is irradiated onto an alignment layer or bank structures are provided, for controlling the alignment of the liquid crystal. The present invention also relates to an exposure apparatus of an alignment layer of a liquid crystal display device and a treatment method of an alignment layer.
2. Description of the Related Art
A liquid crystal display device includes a liquid crystal that is sandwiched between a pair of substrates, and an electrode for applying an electric field to the liquid crystal and an alignment layer for controlling the alignment of the liquid crystal are arranged on the inner surface side of each substrate. The alignment layer is treated for realizing the alignment of the liquid crystal molecules in a predetermined direction. Typically, the alignment layer is rubbed by a fiber material such as rayon, and the liquid crystal molecules are aligned in the rubbing direction. When the alignment layer is rubbed with the fiber material, however, the fiber material scatters, and the liquid crystal panel may be contaminated. Therefore, a technology that can control the alignment of the liquid crystal, without rubbing the alignment layer, has been required.
For example, Japanese Unexamined Patent Publication (Kokai) No. 11-202336 discloses a technology in which the alignment of the liquid crystal is controlled by obliquely irradiating the ultraviolet light onto the alignment layer. In the invention described in this publication, the ultraviolet light is obliquely irradiated onto the alignment layer through a mask having slits while the alignment layer is moved.
Japanese Unexamined Patent Publication (Kokai) No. 11-352489 discloses a technology in which the alignment of the liquid crystal is controlled by disposing bank structures (projection patterns) on the alignment layer. This technology uses a vertical alignment layer, and the liquid crystal molecules are aligned vertically to the alignment layer. At positions where the bank structure (projection pattern) exists, the liquid crystal molecules are aligned perpendicular to the side surface of the bank structure (projection pattern) and generally obliquely to the substrate surface. The bank structure (projection pattern) has side surfaces on either side thereof. The alignment direction of the liquid crystal molecules aligned vertically to one of the side surfaces of the bank structure (projection pattern) is opposite to the alignment direction of the liquid crystal molecules aligned vertically to the other side surface on the opposite side of the bank structure (projection pattern). In this way, alignment division is achieved.
Alignment division means that one pixel is divided into a plurality of regions having different alignments of the liquid crystal. In the case of rubbing, for example, the area of one pixel of the alignment layer is divided into two regions, the first region is rubbed in one direction and the second region is rubbed in the opposite direction. Thus, the liquid crystal molecules located in contact with the first region pretilt in one direction and the liquid crystal molecules located in contact with the second region pretilt in the opposite direction. It is known that viewing angle characteristics can be improved in the liquid crystal display device having such an alignment division.
Alignment division can be easily accomplished if a liquid crystal having negative dielectric anisotropy and an alignment layer having vertical alignment property are used.
When the ultraviolet light is obliquely irradiated onto the alignment layer having a vertical alignment property, alkyl side chains of the alignment layer existing in a plane perpendicular to the irradiation direction of the ultraviolet light absorb the ultraviolet light and are cut off, and alkyl side chains of the alignment layer existing in a plane parallel to the irradiation direction of the ultraviolet light do not absorb the ultraviolet light and remain as such. The liquid crystal molecules are thus aligned in accordance with the remaining alkyl side chains. To achieve alignment division, the ultraviolet light is irradiated obliquely onto one of the regions of the alignment layer in one oblique direction and is irradiated onto the other region of the alignment layer in the opposite oblique direction. In this case, a mask is arranged between the UV light source and the alignment layer so that the ultraviolet light can be irradiated selectively onto one region and the other region.
Various materials are used for the alignment layer. For example, Japanese Unexamined Patent Publication (Kokai) No. 64-004720 discloses a TN type liquid crystal display device using an alignment layer comprising a mixture of polyamic acid and polyimide. In this reference, however, the mixture of polyamic acid and polyimide constitutes a TN type liquid crystal cell.
Further, as the ultraviolet light is irradiated obliquely onto the alignment layer having a vertical alignment property, the liquid crystal is aligned in a direction parallel to the irradiation direction of the ultraviolet light used as the liquid crystal display device. On the other hand, an electrode is disposed with the alignment layer on each substrate. One of the substrates is a TFT substrate having a plurality of pixel electrodes and bus lines. The other substrate is a color filter substrate having a common electrode. The alignment treatment of the alignment layer is conducted in such a fashion that the alignment direction of the liquid crystal is parallel to the bus lines. In this case, a transverse electric field acts between the pixel electrode and the bus line, and a problem develops that this electric field disturbs the alignment of the liquid crystal at the boundary portion between the pixel electrode and the bus line.
In the case where the ultraviolet light is obliquely irradiated onto the alignment layer, the UV light source is disposed at a certain angle to the alignment layer. In the arrangement having such an angle, the distance (optical path length) between the UV light source and the alignment layer varies depending on the position of the alignment layer. Therefore, the intensity of the ultraviolet light irradiated onto the alignment layer varies and thus the tilt angle realized thereby may vary. In consequence, stable alignment cannot be achieved and excellent display cannot be provided as domains appear.
To accomplish alignment division in the alignment control technology using UV irradiation, an exposure apparatus including a UV light source and a mask is used. The mask is arranged between the UV light source and the alignment layer so that the ultraviolet light can be selectively irradiated onto portions of the alignment layer. In the first method of alignment division, the area of the opening of the mask is decreased so that the ultraviolet light having scattering property pass through the opening of the mask. However, this method involves the problem that utilization efficiency of the ultraviolet light is low and the irradiation time of the ultraviolet light must be elongated.
In the second alignment division method, the area of the opening of the mask is set to a size suitable for irradiating a half of the pixel, the UV light source is obliquely arranged relative to the mask and the ultraviolet light is irradiated onto a half of each pixel, and then the UV light source is oppositely obliquely arranged relative to the mask and the ultraviolet light is irradiated onto the remaining half of each pixel. In this case, however, positioning between the mask and the alignment layer becomes more difficult as the size of the pixels become smaller. Also, in a proximity exposure, a gap is provided between the mask and the alignment layer and their positioning is conducted. As the size of the pixels become smaller, however, the gap between the mask and the alignment layer must be decreased, but it is not possible to reduce the space below an allowable value.